The disposal of consumer waste has become a significant problem in many industrialised countries. For example, there are relatively few sites that remain available for landfill in places such as Europe and Japan. A considerable volume of consumer waste is made up of polymeric material, and there has been a concerted effort to introduce polymer recycling strategies to reduce such polymer waste going to landfill. However, unlike other materials such as glass, wood and metal, the recycling of polymers can be problematic. For example, polymer recycling techniques typically require the polymers to be sorted according to their chemical composition. However, due to the diverse array of different commercial polymers it can be difficult to separate polymer materials from the waste stream in this manner. Furthermore, most polymer recycling techniques involve a melt processing stage which can reduce the physical and mechanical properties of the polymer. Recycled polymers therefore tend to have inferior properties and this can limit the range of applications in which they can be employed.
Apart from problems associated with recycling waste polymer materials, the majority of polymers currently being used are derived from petroleum-based products, making their long-term manufacture unsustainable.
In response to these issues, there has been a marked increase in research directed toward developing biodegradable polymers that can at least in part be manufactured using renewable resources. Unlike conventional polymers, biodegradable polymers can be more readily degraded through the action of microorganisms to produce low molecular weight products that present little, if any, environmental concern. Furthermore, through the action of biodegradation the volume occupied by such polymers in waste streams is significantly reduced.
Much of the research to-date in the field of biodegradable polymers has focussed on utilising naturally occurring bio-polymers such as polysaccharides. Perhaps the most widely studied polysaccharide in this regard is starch. Starch is a particularly suitable bio-polymer in that it is derived from renewable resources (i.e. plant products), readily available and relatively inexpensive. However, the physical and mechanical properties of starch in its native form are relatively poor compared with those of conventional petroleum based (i.e. “synthetic”) polymers.
A number of techniques have been developed to improve the physical and mechanical properties of native starch. One approach has involved converting native starch into a thermoplastically processible starch (TPS). For example, PCT/WO90/05161 discloses a process for producing TPS which comprises melt mixing starch having a low water content with a plasticiser such as glycerol. Although the physical and mechanical properties of such TPS polymers are substantially better than native starch, these polymers typically have poor water resistance and can therefore only be used in limited applications.
The water resistance of TPS polymers can be improved by blending these polymers with other thermoplastic polymers such as polyolefins. However, the biodegradability of these TPS polymer blends can be adversely affected due to the fact that polymers that are usually blended with the TPS are relatively non-biodegradable. Furthermore, the physical and mechanical properties of such TPS polymer blends are often quite poor due to the immiscibility of polymers employed in making the blends. In particular, polysaccharides such as starch and TPS are relatively hydrophilic, whereas most synthetic thermoplastic polymers are relatively hydrophobic. Accordingly, melt blending of starch or TPS with other thermoplastic polymers typically results in the formation of a multi-phase morphology having a high interfacial tension which can negatively impact on the physical and mechanical properties of the resulting polymer blend.
Attempts have been made to improve the biodegradability and the physical and mechanical properties of TPS polymer blends. For example, U.S. Pat. No. 5,844,023 discloses a biologically degradable polymer mixture comprising a biodegradable polyester, a TPS and a “polymer phase mediator”. The polymer mixture is said to be readily biodegradable and the polymer phase mediator is said to promote coupling of hydrophobic polyester phase and hydrophilic TPS phase thereby improving the physical and mechanical properties of the polymer mixture. A biodegradable polymer composition disclosed in the US reference is formed through melt mixing a thermoplastic polyester with TPS. In this case, the polymer phase mediator is said to be formed in situ during this melt mixing process through transesterification between some of the polyester and some of the TPS. Formation of the phase mediator in this way is considered difficult to control, and the process is believed to provide a limited reduction in the interfacial tension between the immiscible polymer phases.
Despite representing an advance in the field of biodegradable polymers, due to only a marginal improvement in phase coupling, the physical and mechanical properties of such polyester/TPS blends are still relatively poor compared with conventional petroleum based polymers. To compensate for this, such polyester/TPS blends are typically prepared with quite low levels of starch. However, lowering the starch content of the composition increases its cost and can reduce its biodegradability.
Accordingly, there remains a need to develop alternative biodegradable polymer compositions having good physical and mechanical properties.